Plans for a new bridge across Venice's Grand Canal have plunged the city into a row over what defines heritage.

Architects and conservationists are fighting the city's decision to build a new bridge, reportedly using metal and glass, in place of the wooden Ponte dell' Accademia, one of just four bridges across the canal. The city also stands accused of rushing the project through without consulting experts.

Defenders of the plan have pointed out that the current bridge was built in 1933 – hardly heritage by Venetian standards – that it was conceived as a temporary structure and has already been replaced by a replica in 1985.

The bridge is also a fire hazard and lacks ramps for the disabled, the city claims, all of which will be put right by the planned, €6m (£5.25m) bridge, which will retain a similar form to the existing structure.

But Lidia Fersuoch, the Venice representative of Italian conservation group Italia Nostra, said 78 years was enough to turn the existing bridge into a Venetian landmark. "The bridge now has its own dignity and should be restored," she said. "Venice risks losing a piece of its identity."

Nerves in Venice are still frayed over the construction, against fierce opposition, of a distinctly modern glass and stone bridge by Spanish architect Santiago Calatrava over the Grand Canal near the city's station. A recent study has shown the bridge moves slightly, while critics argue it is high-maintenance and a danger to tourists – some have twisted ankles slipping on the bridge's irregular steps.